The present invention relates to an intervertebral spacer for inserting between two adjacent cervical vertebrae, and to a set of intervertebral spacers comprising two intervertebral spacers of the above-specified type.
The human spine is made up of twenty-four true vertebrae placed one above the other and connected together by pieces of fibrocartilage known as intervertebral disks. These vertebrae constitute three groups: seven cervical vertebrae, twelve dorsal vertebrae, and five lumbar vertebrae. The vertebrae are of different shapes in the three regions (cervical dorsal, and lumbar) of the spine, but they all retain certain general characteristics. Every vertebra comprises:
an enlarged anterior portion known as the vertebral body;
a bony arch that is concave on the anterior side, generally referred to as the posterior arch or neural arch, and that co-operates with the posterior face of the vertebral body to circumscribe an orifice (known as the vertebral foramen) through which the spinal cord passes, said arch being formed on either side by front pedicles and by rear vertebral laminae; and
a middle posterior projection known as the spinous process.
During the life of an individual, it can happen that an intervertebral disk becomes damaged, for various reasons. Under such circumstances, the stresses that act on said disk as a result of relative displacements between the two vertebrae on either side of the disk give rise to pain. Thus, in order to relieve such pain, attempts are made to hold the two vertebrae stationary relative to each other.
Numerous vertebral spacers are already known for placing between two adjacent dorsal or lumbar vertebrae. Such spacers generally present two longitudinally-extending notches formed respectively in their top and bottom faces, each serving to receive the spinous process of one of the two vertebrae. An example of a spacer of that type can be found in the French patent application made public under publication No. FR 2 799 640.
Although that type of prior art spacer is well adapted to the dorsal and lumbar regions, it is nevertheless impossible to use it with cervical vertebrae, in part for the following reasons.
Firstly, the spine in the cervical region is convex in the anterior direction, known as cervical lordosis, which means that the spinous processes of the vertebrae are moved towards one another. As a result, these processes are so close together (some processes touch one another) that it is not possible to install a spacer between them.
Secondly, whereas in the dorsal or lumbar region the relative movements between two adjacent vertebrae consist mainly in the vertebrae moving apart or towards each other, where these movements are caused respectively by the spine flexing or extending, in the cervical region, the movements involved are more complex. Thus, there are movements of one vertebra turning relative to another caused by twisting the spine, or movements caused by the spine being flexed laterally (i.e. the spine moves laterally away from the sagittal plane), which can be resolved into a rotation and an asymmetrical movement apart between one vertebra and another, and these movements are much more marked in the cervical region than in the dorsal or lumbar regions.